Differences in hydration state of nucleus and cytoplasm of the amphibian oocyte

Calculation of the relative metastabilities of proteins in subcellular compartments of Saccharomyces cerevisiae

Background

Protein subcellular localization and differences in oxidation state between subcellular compartments are two well-studied features of the the cellular organization of S. cerevisiae (yeast). Theories about the origin of subcellular organization are assisted by computational models that can integrate data from observations of compositional and chemical properties of the system.

Presentation and implications of the hypothesis

I adopt the hypothesis that the state of yeast subcellular organization is in a local energy minimum. This hypothesis implies that equilibrium thermodynamic models can yield predictions about the interdependence between populations of proteins and their subcellular chemical environments.

Testing the hypothesis

Three types of tests are proposed. First, there should be correlations between modeled and observed oxidation states for different compartments. Second, there should be a correspondence between the energy requirements of protein formation and the order the appearance of organelles during cellular development. Third, there should be correlations between the predicted and observed relative abundances of interacting proteins within compartments.

Results

The relative metastability fields of subcellular homologs of glutaredoxin and thioredoxin indicate a trend from less to more oxidizing as mitochondrion – cytoplasm – nucleus. Representing the overall amino acid compositions of proteins in 23 different compartments each with a single reference model protein suggests that the formation reactions for proteins in the vacuole (in relatively oxidizing conditions), ER and early Golgi (in relatively reducing conditions) are relatively highly favored, while that for the microtubule is the most costly. The relative abundances of model proteins for each compartment inferred from experimental data were found in some cases to correlate with the predicted abundances, and both positive and negative correlations were found for some assemblages of proteins in known complexes.

Conclusion

The results of these calculations and tests suggest that a tendency toward a metastable energy minimum could underlie some organizational links between the the chemical thermodynamic properties of proteins and subcellular chemical environments. Future models of this kind will benefit from consideration of additional thermodynamic variables together with more detailed subcellular observations.

Experimental study of intracellular ice growth in human umbilical vein endothelial cells

Study of the intracellular ice formation (IIF) and growth is essential to the mechanistic understanding of cellular damage through freezing. In the aid of high speed and high resolution cryo-imaging technology, the transient intracellular ice formation and growth processes of the attached human umbilical vein endothelial cells (HUVEC) were successfully captured during freezing. It was found that the intracellular ice nucleation site was on the cell membrane closer to the nucleus. The ice growth was directional and toward the nucleus, which covered the whole nucleus before growing into the cytoplasm. The crystal growth rate in the nucleus was much larger than that in the cytoplasm, and its morphology was influenced by the cooling rate. During the thawing process, small crystals fused into larger ones inside the nucleus. Moreover, the cumulative fraction of the HUVEC with IIF was mainly dependent on the cooling rate not the confluence of the cells attached.

Progesterone binding to plasma membrane and cytosol receptors in the amphibian oocyte

We have found a single class of progesterone binding sites at the amphibian oocyte plasma membrane, whereas two progesterone receptor forms, similar to those in chick and human, are present in the cytosol. In this study both plasma membranes and 105,000 x g cytosol from Rana pipiens oocytes were photoaffinity labeled with the synthetic progestin [3H]R5020. SDS-polyacrylamide gel electrophoresis of the photolabeled proteins in the oocyte cytosol indicate that the two forms have molecular weights essentially identical to that found for human breast tissue and chick oviduct, i.e., 80 and 110 kDa, and that the forms were present in approximately equimolar ratios. In contrast, the plasma membrane form is present as a single 110 kDa species and accounts for at least 50% of the total 110 kDa species. The presence of large amounts of the 110 kDa protein in both membrane and cytosol suggests that the plasma membrane receptor may not be unique, and that the 110 kDa form may function both in membrane and cytosol and/or that part of the cytosolic 110 kDa form represents progesterone receptor in the process of being transported to or from the plasma membrane.